The present invention relates, in general, to the field of radio frequency ("RF") identification ("ID") systems. More particularly, the present invention relates to a system and method for initiating communications between a controller and a selected one or other subset of multiple transponders in a common RF field. The system and method of the present invention is of particular utility in conjunction with the Racom RFM256 passive, proximity RF read/write card transponder ("RF/ID Tag") and an associated RFC100 CORE reader/writer electronics module controller available from Racom Systems, Inc., 6080 Greenwood Plaza Boulevard, Englewood, Colo., 80111 or other similar devices.
The Racom RFM256 is a passive RF transponder incorporating a non-volatile memory element which is powered by inductive coupling to a proximately located RF controller. The Racom communication system utilizes the controller to frequency shift key ("FSK") the powering RF signal to cause the transponder to either read data from, or write data to, the non-volatile memory and transmit the same back to the controller utilizing phase shift keying ("PSK") modulation in response to specific commands from the controller. The Racom communications system may operate in either full duplex or half duplex modes with the controller causing the transponder to simultaneously write data to its non-volatile memory array as the contents are read out and transmitted to the controller by complementing selected bits thereof. The Racom RFM256 transponder utilizes a ferroelectric random access memory ("FRAM.RTM.") non-volatile integrated circuit memory array manufactured using a proprietary lead-zirconate-titanate ("PZT") ceramic thin film process and is available from Ramtron International Corporation, Colorado Springs, Colo., the assignee of the present invention.
In operation, the Racom communication system utilizes FSK modulated RF signals transmitted from the controller to the transponder from which the latter derives it power. The use of FSK modulation allows the average signal power to remain at a maximum as opposed to other modulation techniques such as amplitude shift keying ("ASK") or pulse modulation techniques and the transponder is able to demodulate the FSK encoded signal without the incorporation of an on-chip oscillator frequency reference. In the current Racom RFM256 transponder system, a controller modulates a data signal between 125 KHz and 116.3 KHz to send a four bit time synchronization block, a start bit, memory address and command bits which are operative to cause the transponder to read: a) only the word at such address or b) the entire contents of the memory beginning at such address.
The RFM256 transponder, upon power up by entering the electromagnetic field of the associated controller, continuously transmits a configuration word to the controller utilizing a coherent PSK modulated signal of 62.5 KHz derived from the 125 KHz FSK RF input.
Utilizing the combination of FSK and PSK modulation techniques, in conjunction with a ferroelectric memory array, allows the transponder "write" range to be the same as the "read" range and effectively precludes interference between read and write commands or other transponders which may be within the RF signal range of the controller. Each RFM256 transponder configuration word includes a sync block and start bit corresponding with those portions of the controller command word as well as the number of bits designating the memory words which have been previously "locked" from further writes by the controller. A number of "hard-wired" mask bits and type bits are utilized to identify a particular transponder, or other subset of multiple transponders, to the controller and distinguish it from others. These or other memory locations within the ferroelectric memory array may be utilized to establish a unique ID number for a given transponder or a uniquely common ID number for a selected subset of multiple transponders with which the controller may desire to communicate.
The Racom transponder and communications system may be operated in either half duplex or full duplex mode. In the former instance, the transponder will transmit the requested words from memory in response to a "read" command beginning at the words specified in the word address portion of the command word. Utilizing this protocol, the controller must first know the contents of the transponder memory before initiating a "write" which operation is effectuated by sending a burst of 116.3 KHz RF power in synchronization with each bit read out of the transponder memory that is desired to complement. Typically, the controller will then initiate another "read" of the relevant portions of the transponder memory to verify that the write operation has been properly effectuated.
Alternatively, the communication system may be operated in full duplex mode whereby the read/write/verify operations are effectuated in a single pass operation. In this mode, the controller will complement the selected bits of the memory words as they are read out by the transponder and verify that the data was written properly during each of the single bit times.